Various methods and techniques for dealing with nitrogen in natural gas liquefaction are known. Some examples include U.S. Pat. Nos. 2,500,129 to Laverty et al.; 2,823,523 to Eakin et al.; 3,559,418 to Hoffman; 3,874,184 to Harper et al.; 4,225,329 to Bailey et al.; and 5,036,671 to Nelson et al. Most of these involve fractionation and/or separation of a nitrogen rich vapor stream from a partially condensed natural gas stream.
Recent advances in the manufacture of plate fin heat exchangers now permit the use of such devices in place of conventional distillation columns in some cryogenic processes including air separation; recovery of hydrogen, ethylene, natural gas liquids and liquefied petroleum gases; and purification of carbon dioxide. Also known as reflux exchangers, both heat and mass transfer operations can be simultaneously effected at high efficiency. A reflux heat exchanger typically has a high ratio of surface area to volume for a light, compact design preferably operating with a minimum temperature driving force of only 2.degree. to 3.degree. C.
A reflux exchanger includes adjacent passages for introducing feed and heat transfer fluids. A liquid feed stream preferably is introduced for downward gravity flow through a feed passage and a heating fluid flows upward through an adjacent heat transfer passage so that the streams are countercurrent to each other. Heat transferred to the downflowing stream effects vaporization of at least part thereof. Vapor thus formed rises up through the same passages as the feed stream to strip the liquid phase of the lightest components. The feed vapor phase is then withdrawn overhead from the feed passage.
In this arrangement, the reflux exchanger resembles the stripping section of a distillation column. However, important differences are evident. Heat exchange coincident with separation along the entire length of the unit permits the driving forces for both heat and mass transfer to remain small for enhanced thermodynamic efficiency. Because the driving forces are small, temperature and compositional differences between vapor and liquid phases more closely represent a reversible thermodynamic process. The reflux exchanger is thus analogous to a multistage stripper having a feboiler at each stage.
A reflux exchanger as a multistage stripper offers a few other benefits over an ordinary distillation column as well. In an ordinary partial vaporization (stripping) process, the feed is heated to a sufficiently high temperature to ensure that most of the lighter components are vaporized out and recovered. This can result in a relatively large amount of unwanted heavier components being vaporized into the vapor phase. In contrast, a reflux exchanger with a lower average reboil temperature has lesser amounts of vaporized heavy components. Consequently, the heating load is reduced because of the reduction in the heat load for reboil. Alternatively, for the same reboil load, better recoveries can be achieved.
It can be seen that for a vapor feed stream, a similar exchanger can be analogously employed as a multistage rectifier. A coincident cooling source at each stage condenses the feed and refluxes the vapor.
A general overview of a plate-fin heat exchanger and the use thereof in natural gas processing is disclosed in Finn, A., Chemical Engineering, Vol. 101, No. 5, pp. 142-147, May 1994.
Costain Oil, Gas & Process, Ltd. Plate Fin Exchanger Bulletin of 1989, pgs. 5-9, describes sizing calculations used to design a plate-fin heat exchanger.
U.S. Pat. No. 3,203,191 to French describes a gas liquefaction process employing an expander to lower energy requirements.
U.S. Pat. No. 4,334,902 to Paradowski describes a process for liquefying natural gas by cooling the gas with the vapor from a liquid coolant subcooled after expansion thereof in the liquid condition wherein the vapor simultaneously subcools the liquefied coolant. The subcooled high pressure liquid coolant is expanded in a hydraulic turbine.